Diesel engines are efficient, durable and economical. Diesel exhaust, however, can harm both the environment and people. To reduce this harm, governments, such as the United States and the European Union, have proposed stricter diesel exhaust emission regulations. These environmental regulations require diesel engines to meet the same pollution emission standards as gasoline engines. Typically, to meet such regulations and standards, diesel engine systems require equipment additions and modifications.
For example, a lean burning engine provides improved fuel efficiency by operating with an amount of oxygen in excess of the amount necessary for complete combustion of the fuel. Such engines are said to run “lean” or on a “lean mixture.” However, the increase in fuel efficiency is offset by the creation of undesirable pollution emissions in the form of nitrogen oxides (NOx). Nitrogen oxide emissions are regulated through regular emission testing requirements. One method used to reduce NOx emissions from lean burn internal combustion engines is known as selective catalytic reduction (SCR). When used to reduce NOx emissions from a diesel engine, selective catalytic reduction involves injecting atomized urea into the exhaust stream of the engine.
The main components of an SCR system are a tank for storing Diesel Exhaust Fluid (DEF) and an injection system for delivering the DEF into the exhaust, as well as an SCR catalytic chamber for the NOX reduction to occur. An onboard diagnostic system monitors the DEF level and indicates when the storage tank needs to be refilled. However, while the DEF is a non-hazardous, high-purity, colorless solution containing 32.5% urea, its high water content (over 65% deionized water) makes DEF very susceptible to freezing in the DEF tank at cold ambient temperatures and difficult to vaporize in cool exhaust streams.
Because the fluid cannot be sprayed if it is frozen in the DEF tank and poor vaporization in cool exhaust streams leads to deposit formation, DEF is unusable in these conditions. Accordingly, in order to be in compliance with emission standards, alternate dosing is needed during such times. The present system and methods solve these and other problems associated with prior art NOx reduction systems.